The present invention relates to conjugates of biologically active compounds, such as proteins or peptides, and fluorine-labelled sugars, in particular 5-fluoro-5-deoxy or 3-fluoro-3-deoxy pentoses, processes for making these and the use of such conjugates, particularly those comprising 5-18F-5-deoxypentoses or 3-18F-3-deoxypentoses, in imaging, in particular positron emission tomography imaging.